[Explore sites]: Modify backend tasks to read from new activity table

Bug: 918436
Change-Id: I3994b6eb937e4e676467380205bfdca522e947c3
Reviewed-on: https://chromium-review.googlesource.com/c/1400349
Commit-Queue: Cathy Li <chili@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622564}
10 files changed